This application is for a one-year study to examine the role of motion sickness susceptibility in height phobia. Contemporary models of phobia suggest that some specific phobias are based on an underlying response tendency that constitutes a vulnerability to the development of a pattern of fear and avoidance to a given class of stimuli. Recent studies have found a relationship between vestibular dysfunction and panic disorder; preliminary results from the PI's laboratory indicate that individuals with height fears report greater susceptibility to motion sickness than non-fearful controls. To date, no studies have used an actual motion sickness induction procedure as a direct measure of susceptibility, nor have any studies determined if motion sickness susceptibility distinguishes height phobia from other specific phobias. Accordingly, the proposed study will employ the optokinetic drum, a widely used method of inducing motion sickness, to compare the susceptibility of individuals with height phobia with that of normal controls and individuals with animal phobias. The optokinetic drum is a cylinder whose interior surface is patterned with alternating vertical black and white stripes; the subject is seated inside the drum while it is rotated, creating an illusory sense of rotation in most subjects, and varying degrees of motion sickness in susceptible subjects. During this procedure, subjects periodically report their overall level of discomfort on an 11 point scale, specific sensations of motion sickness, and symptoms of anxiety. The central index of susceptibility is the latency to reach a level of 5 on the 11 point scale of overall discomfort. It is predicted that subjects with height phobias will show shorter latencies to reach this level of discomfort than either normal controls or subjects with animal phobias. Multiple regression analyses will be used to examine the interrelationship between anxiety sensitivity, motion sickness susceptibility, and height fears. These data have potential implications for understanding one of the underlying response processes in height fears and for the identification of common dimensions among the anxiety disorders, and for the treatment of height phobias.